The present invention relates to power converter systems, and, more specifically, to power conversion systems capable of providing a desired output waveform over a wide range of input and load conditions.
In general, power conversion systems comprising a generator and an energy source, such as a motor or turbine, are well known. The generator typically comprises a rotor and stator arranged for relative rotation. Generally, the rotor is driven by the energy source, often mounted on the shaft of the motor. The rotor typically generates a magnetic field (using either permanent magnets or windings), which interacts with windings maintained on the stator. As the magnetic field intercepts the windings, an electrical current is generated. The induced current is typically applied to a bridge rectifier, sometimes regulated, and provided as an output. In some instances, the rectified signal is applied to an inverter to generate an AC output.
Generators which use permanent magnets to generate the requisite magnetic field tend to be lighter and smaller than traditional wound field generators. However, the power supplied by permanent magnet generator has historically been difficult to regulate or control. The voltage supplied by the generator varies significantly according to the speed of the rotor. In addition, the voltage tends to vary inversely with the current delivered, i.e., as the current increases to a given load, the voltage drops.
For example, it is desirable to employ permanent magnet generators in electric welders. However, electric welders typically require a particular current to voltage relationship. For example, arc welders require an inverse slope of current to voltage, whereas metal inert gas (MIG) welders (wire feed welders) require a constant voltage and variable current and tungsten inert gas (TIG) welders require a constant current and variable voltage. Since permanent magnet generator's outputs are dependent upon motor speed, they are typically not suitable for electric welder applications. This is particularly true with respect to multipurpose welders that provide a plurality of electrical welding types.
It is also particularly desirable that a generator be able to accommodate wide and rapidly occurring variations in load, and hence output current. For example, when an incandescent lamp with a cold filament is "plugged in" to the generator, the generator is presented with extremely low resistance, resulting in an extremely high current, often in excess of ten times the average output current draw. In the absence of special provisions, components typically must be rated for the anticipated peak currents rather than the much lower magnitude of the average output current. The requirement for components rated for peak voltages much higher than the average output current tends to add considerable expense to the generator.
In addition, the load encountered by the generator is often inductive in nature, e.g., an induction motor. Accordingly, the phase of the current tends to lag the phase of the voltage. However, the switching devices in the inverter bridge are typically responsive to the voltage wave form, and often shut off, i.e., are rendered nonconductive, before the relevant portion of the current cycle has been completed. Accordingly, otherwise available energy is effectively lost. For example, when the load is an inductive motor, magnetism, and thus torque, ceases at the point that the current ceases to flow.
Moreover, generators capable of starting motor vehicles are typically ungainly, and heavy, weighing on the order of 180 pounds or more.
The present invention provides a particularly advantageous power converter, suitable for use in an engine powered generator system.
A signal simulating a desired AC waveform is produced by a converter circuit at first and second converter output terminals. The converter circuit, responsive to respective switching signals applied thereto, selectively effects current paths between a juncture node and one of the first and second converter output terminals and between a common rail and the other of the first and second converter output terminals. A controller for selectively generates control signals to the converter circuit and to a mechanism for varying the magnitude of the juncture node voltage, to create a predetermined waveform at the converter output terminals simulating the desired AC waveform.
In accordance with one aspect of the present invention, a simulated sine wave is provided. For example, in one embodiment, control signals are generated in sequence to (a) cause the converter to effect a current path between the juncture node and the first converter output terminal and between the common rail and the second converter output terminal, with the magnitude of the juncture node voltage initially at a first level; (b) increase magnitude of the juncture node voltage to a second level; (c) change in the magnitude of the juncture node voltage back to the first level; (d) cause the converter to effectively break at least one of the current paths between the juncture node and the first converter output terminal and between the common rail and the second converter output terminal; and (e) cause the converter to effect a current path between the juncture node and the second converter output terminal and between the common rail and the first converter output terminal. By way of further example, in another embodiment, a charging path to a capacitor is effected during dead time periods when there is no current path between the juncture node and one of the first and second converter output terminals, and a discharge path from the capacitor to the juncture node selectively effecting during a predetermined portion of the periods when one of the power switch circuits effect a current path between the juncture node and one of the first and second converter output terminals. By way of yet another example, in a further embodiment, a current path is selectively effected from the juncture node through a capacitance to the common rail to vary the magnitude of the juncture node voltage. In some applications, the capacitance may be controllably discharged to facilitate rapid generation of a relatively low magnitude juncture node voltage.
Another aspect of the present invention provides an accommodation for inductive loads. For example, in one embodiment, control signals are generated to cause the converter to effectively break the current path between the juncture node and the first converter output terminal, and thereafter, but before effect a current path between the juncture node and the second converter output terminal and between the common rail and the first converter output terminal cause the converter to effectively break the current path between the common rail and the second converter output terminal.
In accordance with another aspect of the present invention, power dissipation during the switching interval is minimized. For example, in one embodiment, the power switching devices are quickly driven into a saturated state when the associated control signal changes state.